If it was tested today, it probably wouldn't get 5 stars anymore, due to pedestrian safety. That is why the new Volvo has a bonnet air bag.

I am not sure if it will become a requirement in the star ratings.
The 1er has a blunt front, recessed wipers to assist the person from getting serious injury in low speed impacts.

It would be a better if all vehicles had the same road height front and rear /safety chassis rails to allow the impact zones to match that of the colliding vehicle.

Pedestrian safety
'Don't step out in front of a car' works best.

How many more air bags can they fit to a car?

Volvo's bonnet air bag (windscreen bag would be a more apropriate name) is only effective at speeds up to 30 kph and does not stop lower body injury and they qualify it about the amount of likely head injury dependant upon the angle of impact.
Jaguar has the bonnet popper that operates like a dampened trampoline at speeds up to 40 kph to slow down the impact rate.

The problem with these devises is like the ones Mercedes fitted to some of their buses (front buffer bags) ten years ago. Reasonably effective at stopping injury in the initial impact until it bounces the person into the path of a vehicle coming the other way. In which case the person is now horizontal at widscreen height or down on the road and under its wheels.